1. Field of the Invention
The present invention relates to an NMR probe for use in an NMR spectrometer and, more particularly, to an NMR probe permitting observation and irradiation of numerous kinds of nuclei.
2. Description of Related Art
An NMR spectrometer is an instrument for analyzing a molecular structure by irradiating a sample placed within a static magnetic field with an RF signal, then detecting a feeble RF signal (NMR signal) emanating from the sample, and extracting information about the molecular structure contained in the signal.
FIG. 1 is a schematic block diagram of the NMR spectrometer. The spectrometer has an RF oscillator 1 producing an RF signal. The RF signal is controlled in terms of phase and amplitude by a phase controller 2 and an amplitude controller 3 and sent to a power amplifier 4.
The RF signal is amplified to an electric power necessary to excite an NMR signal by the power amplifier 4 and sent to an NMR probe 6 via a duplexer 5. Then, the signal is applied to the sample from a detection coil (not shown) placed within the NMR probe 6.
After the RF irradiation, a feeble NMR signal emanating from the sample is detected by the detection coil (not shown) placed within the NMR probe 6 and sent to a preamplifier 7 again via the duplexer 5. In the preamplifier 7, the signal is amplified to a signal level at which the signal can be picked up.
A receiver 8 converts the frequency of the RF NMR signal amplified by the preamplifier 7 to an audio frequency that can be converted into a digital signal. At the same time, the receiver controls the amplitude. The NMR signal converted into the audio frequency by the receiver 8 is converted into a digital signal by an analog-to-digital data converter 9 and sent to a control computer 10.
The control computer 10 controls the phase controller 2 and amplitude controller 3, Fourier-transforms the NMR signal accepted in the time domain, automatically corrects the phase of the Fourier-transformed NMR signal, and then displays the NMR signal as an NMR spectrum.
There are many kinds of RF waves applied to the NMR probe 6. In particular, RF waves corresponding to the resonant frequency of any one of nuclear species as shown in the following Table 1 are applied to the NMR probe.
TABLE 1Resonant Frequency at 18 Tesla (MHz)Kinds ofResonantKinds ofResonantKinds ofResonantNucleiFrequencyNucleiFrequencyNucleiFrequency3H80027Al19515N761H75013C18935Cl7419F70679Br18814N54205Tl43329Si14939K3531P304199Hg13499Ru357Li2922H115183W31119Sn2806Li110103Rh2411B24117O102
In Table 1, the chemical symbols on the left side of each column of the table indicate the kinds of nuclei under observation, while the numerical values on the right side indicate the resonant frequencies (in MHz) of the observed nuclei in a case where they are placed within a static magnetic field of 18 Tesla (T).
In the following description, let HF be the RF waves of frequencies corresponding to the resonant frequencies of 3H nuclei to 19F nuclei. Let LF be RF waves having frequencies lower than the resonant frequency of 205Tl. LOCK is an abbreviation of RF waves having a lock frequency using deuterium nuclei.
FIGS. 2A and 2B show one example of an NMR probe of a type called the inverse probe. In this type of NMR probe, there are two detection coils which are disposed to surround a sample tube.
One of the two coils is an inner coil 11 for observing HF. The inductance of the inner coil is so set that it doubly resonates with HF and LOCK. The other is an outer coil 12 for applying LF for decoupling purposes. The inductance of the outer coil is so set that LF having a frequency capable of being varied over a wide range can be covered efficiently. In this probe, HF is observed with the inner coil 11 closer to the sample under investigation and, therefore, HF is detected with enhanced sensitivity.
The inner coil 11 and outer coil 12 are so disposed that RF magnetic fields produced by them are perpendicular to each other in direction. This probe is termed the HX probe due to the fact that RF waves injected into the inner and outer coils are various HF nuclei (typified by hydrogen nuclei and hereinafter referred to as H nuclei) and various LF nuclei (hereinafter referred to as X nuclei), respectively.
An inner coil subassembly (indicated by the circuit diagram in the right top portion of FIG. 2B) including the inner coil 11 has an HF input/output terminal 13 and a LOCK input/output terminal 14. The inner coil is matched and tuned to HF by a matching variable capacitor VC1 for HF and by a tuning variable capacitor VC3 for HF. The inner coil is matched and tuned to LOCK by a matching variable capacitor VC2 for lock and by a tuning variable capacitor VC4 for lock.
An outer coil subassembly (indicated by the circuit diagram in the right bottom portion of FIG. 2B) including the outer coil 12 has an LF input/output terminal 15. The outer coil is matched and tuned to LF by a matching variable capacitor VC5 for LF and by a tuning variable capacitor VC6 for LF.
A capacitor C1 cooperates with the variable capacitor VC1 to match the inner coil to HF. Another capacitor C2 cooperates with a coil L1, the inner coil 11, a coil L2, and the variable capacitor VC4 and resonates the inner coil at the LOCK frequency. The coil L1 keeps high the impedance of the inner coil 11 at the HF frequency and prevents the HF frequency from escaping to ground. The coil L2 prevents the HF frequency from leaking to the LOCK input/output terminal 14 and to ground.
In contrast, FIG. 3 shows one example of an NMR probe of a type known as the tunable probe. Because the inner coil side is tunable, the probe is also referred to as the TH probe. This type of NMR probe also has two detection coils which are disposed to surround a sample tube. The difference with the inverse probe (HX probe) is only that the inner coil 11 is used to observe LF while the outer coil 12 is used to apply radiation for decoupling HF, i.e., the positional relationships of the inner and outer coils are reversed. In this probe, LF is observed with the inner coil 11 closer to the sample under investigation and so LF is detected with enhanced sensitivity. The inner coil 11 and outer coil 12 of the TH probe are also arranged such that the magnetic fields produced by the coils are perpendicular to each other in direction.
In either type of probe, an NMR locking function is often attached. In almost all cases, NMR lock is used, employing resonance frequency of deuterium nuclei (2H nuclei). Because the resonant frequency of deuterium nuclei is about one-sixth as high as the HF frequency, deuterium nuclei are often doubly resonated with an HF coil because of ease of realization of the circuit configuration. If one attempts to achieve the NMR lock using an LF coil, the circuit configuration cannot be easily realized because the resonant frequency of deuterium nuclei is included within the range of LF frequencies. In practice, an example in which the LF coil resonates the probe with LOCK is not general. An HF coil resonates the probe with LOCK (i.e., double resonance).
The prior art probe types, inner and outer coils, and assignments of HF, LF, and LOCK are summarized in Table 2.
TABLE 2Probe TypeInner CoilOuter CoilHX an inverseHF (observation channel) LOCKLFTH a tunableLF (observation channel)HFLOCK
In the prior art probe (see JP2003-121523), the coil for observing and irradiating HF is clearly distinguished from the coil for observing and irradiating LF, regardless of whether it is an inverse probe or a tunable probe. Therefore, if one attempts to make observations with the inner coil having higher detection sensitivity, plural probes must be prepared according to whether the resonant frequency of the nuclei of interest is HF or LF. Hence, there is the problem that the used probe must be exchanged according to the purpose.